Motor-operated power steering device

ABSTRACT

An electrically driven power steering apparatus is structured such that a large impact on a rack shaft ( 22 ) occurs upon an impingement of a steering stopper, and, even when an axial force acting on a ball bearing ( 25 ) of a presser member ( 30 ) becomes zero, a rotation of the presser member ( 30 ) is hindered by caulking c at a thin plate member ( 30   b ). Therefore, the presser member ( 30 ) does not slacken, and a predetermined axial force can be given again to the ball bearing ( 25 ) when the impact dissipates. The thus structured power steering apparatus exhibiting, though the cost is low, an excellent shockproof characteristic and capable of preventing the bearing from coming off a ball screw nut, can be provided.

TECHNICAL FIELD

[0001] The present invention relates to a vehicle power steeringapparatus using an electric motor.

BACKGROUND ARTS

[0002] One known type of an electrically driven power steering apparatusof a vehicle is that a ball screw nut into which a rack shaft of arack-and-pinion steering apparatus is inserted, is rotated by anelectric motor coaxial with the rack shaft, and a rotational outputthereof is converted into a thrust in the longitudinal direction of therack shaft through a ball screw mechanism.

[0003] In this type of electrically driven power steering apparatus, theball screw nut is so supported as to be rotatable with respect to ahousing, which involves the use of bearings. These bearings are disposedat both side ends of the ball screw nut and provided with pressermembers for preventing the bearings from coming off the ball screw nut.

[0004] The presser member is formed with a female thread meshing with amale thread formed on an outer periphery of the ball screw nut. Thepresser member is thus screwed to the ball screw nut and thus preventsthe bearing from coming off.

[0005] By the way, for example, in a general type of electrically drivenpower steering apparatus, a steering wheel, when rotating on, finallycomes into contact with a steering stopper with the result that furtherrotations are hindered. If a driver energetically turns the steeringwheel, however, an excessive impingement upon the steering stopper mightoccur. In such a case, an impact as strong as approximately 98.1 kN (10tons) at the maximum might occur on the rack shaft.

[0006] Such a strong impact, when occurred, might exceed an initialaxial force of the presser member for preventing the bearing from comingoff the ball screw nut, and, when traveling oscillations are added, thepresser member might slacken.

[0007] If the presser member is screwed to the ball screw nut with astrong toque in order to increase the initial axial force for preventingthe presser member from slackening, a ball rolling path formed in theball screw nut might deform large enough to cause a decline of functionof the ball screw nut.

[0008] On the other hand, there arises a problem of figuring out whatmode a backlash of balls in the rolling path formed between the ballscrew nut and the ball screw shaft should be adjusted.

[0009] Further, the electrically driven power steering apparatus usingthe electric motor has been used in terms of saving a fuel cost over therecent years. In the electrically driven power steering apparatus, theelectric motor supplied with the electric power from a battery suppliesan assistive steering force, and therefore the power may not be takenout of an internal combustion engine directly. Hence, the fuel cost canbe saved.

[0010] By the way, for instance, in the rack-and-pinion typeelectrically driven power steering apparatus, if a traveling wheelcollides with a paved step of sidewalk during traveling of the vehicle,a large impact might be transferred to the rack shaft from tie rods.Such an impact is transferred tracing back a steering force transferroute and might exert a strong stress on the respective members. Whileon the other hand, a sufficient rigidity against such an impact isensured, a problem is that the respective members increase both in sizesand weights.

DISCLOSURE OF THE INVENTION

[0011] It is an object of the present invention to provide anelectrically driven power steering apparatus exhibiting, though the costis low, an excellent shockproof characteristic and capable of preventinga bearing from coming off a ball screw nut.

[0012] It is another object of the present invention to provide anelectrically driven power steering apparatus capable of reliving animpact.

[0013] An electrically driven power steering apparatus according to thepresent invention comprises a housing, a ball screw shaft extendingwithin the housing and connected to a steering mechanism, an input shaftto which a steering force is inputted, an output shaft for receiving thesteering force from the input shaft and outputting the steering force tothe ball screw shaft, a torque sensor for detecting a torque transferredbetween the input shaft and the output shaft, a motor including a rotor,and a ball screw nut for giving a force in an axial direction to theball screw shaft by receiving a rotational force from the motor, whereinan elastic member deforming and thus capable of absorbing an impactinputted from the side of the ball screw shaft, is disposed on a powertransmission route between the ball screw shaft and the rotor of themotor. Such an impact can be relieved without bringing about increasesboth in sizes and weights of the respective members.

[0014] Further, in a case where the ball screw shaft is integral withthe rack shaft, a load occurred on the traveling wheel in a way thatdepends on a condition of the surface of the traveling road, istransferred to the rack shaft, and consequently the rack shaft, i.e.,the ball screw shaft might displace corresponding to this load in theaxial direction. If the elastic member is not provided, however, such adisplacement is hindered by a friction and an inertia of the motor.According to the present invention, however, the elastic member deforms,whereby the rack shaft can displace in the axial direction without beingaffected by the friction and inertia. As a result, the displacement istransferred to the rack shaft, a pinion, a steering shaft and a steeringwheel, whereby the driver can be precisely informed of so-called roadinformation such as the load occurred on the tire depending on thecondition of the road surface, fluctuations thereof and so on.

[0015] Further, it is that the elastic member is disposed between theball screw nut and the rotor of the motor, and the impact inputted fromthe side of the ball screw shaft is absorbed by a torsional dampereffect.

[0016] Moreover, a displacement limiter for limiting a predetermined orlarger quantity of deformation of the elastic member is provided andconstructed of a recessed portion formed in one of the rotor of themotor and the ball screw nut and a protruded portion formed on theother, and the protruded portion, when the elastic member deforms by thepredetermined quantity, engages with the recessed portion. With thiscontrivance, an excessive deformation of the elastic member isrestrained, and a damage to this elastic member can be prevented.

[0017] An electrically driven power steering apparatus according to thepresent invention comprises a housing, a ball screw shaft extendingwithin the housing and connected to a steering mechanism, an input shaftto which a steering force is inputted, an output shaft for receiving thesteering force from the input shaft and outputting the steering force tothe ball screw shaft, a torque sensor for detecting a torque transferredbetween the input shaft and the output shaft, a motor including a rotor,and a ball screw nut for giving a force acting in an axial direction tothe ball screw shaft by receiving a rotational force from the motor,wherein an elastic member deforming and thus capable of absorbing animpact inputted from the side of the ball screw shaft, is disposed on asupport portion of the ball screw nut. Such an impact can be relievedwithout bringing about the increases both in sizes and weights of therespective members.

[0018] Furthermore, it is preferable that the elastic member is disposedbetween the housing and a bearing for supporting the ball screw nut soas to be rotatable with respect to the housing or between the bearingand the ball screw nut.

[0019] The bearing and the housing relatively move in the axialdirection corresponding to the axis-directional deformation of theelastic member, and there is provided a displacement limiter forlimiting a predetermined or larger quantity of deformation of theelastic member by limiting the relative movements of the bearing and thehousing in the axial direction. With this contrivance, an excessivedeformation of the elastic member is restrained, and a damage to thiselastic member can be prevented.

[0020] The rotor of the motor and the ball screw nut are connected by anengagement between a female spline and a male spline of which at leastone toothed surface is coated with a resin. If the impact istransferred, an emission of butting noises can be effectivelyrestrained.

[0021] It is preferable that the displacement limiter limits thepredetermined or larger quantity of deformation of the elastic member at40% or smaller of a maximum steering force exhibited by the motor.

[0022] It is also preferable that a natural oscillation frequency of asystem constructed of the rotor, the ball screw nut and the elasticmember is set to 7 Hz or higher.

[0023] An electrically driven power steering apparatus according to thepresent invention comprises a housing, a ball screw shaft extendingwithin the housing and connected to a steering mechanism, a motor havinga rotor, a ball screw nut, connected to the rotor of the motor, forconverting a rotational force of the rotor into a force acting in anaxial direction and transferring the same force to the ball screw shaft,a bearing for supporting the ball screw nut to as to be rotatable withrespect to the housing, and a presser member, screwed to the ball screwnut, for thus pressing the bearing against the ball screw nut, whereinthe presser member includes a connection member for connecting thepresser member to the ball screw nut so that the presser member and theball screw nut are unable to relatively rotate. If a large impact on theball screw shaft occurs upon an impingement of a steering stopper, andeven when an axial force acting on the ball bearing of the pressermember becomes zero, a rotation of the presser member is hindered.Therefore, the presser member does not slacken, and a predeterminedaxial force can be given again to the bearing when the impactdissipates.

[0024] Further, it is preferable that the connection member connects thepresser member to the ball screw nut so as to be unable to relativelyrotate by use of a shearing force of a resinous material.

[0025] Moreover, it is preferable that the connection member connectsthe presser member to the ball screw nut so as to be unable torelatively rotate by use of a frictional force.

[0026] The presser member is screwed to the ball screw nut and thusadjusts a backlash of the balls within the ball screw nut. Hence, thereis no necessity of providing a backlash removing means separately, andthe electrically driven power steering apparatus can be provided at alow cost.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a schematic view showing a configuration of anelectrically driven power steering apparatus in a first embodiment;

[0028]FIG. 2 is a sectional view taken in an axial direction, showingthe periphery of a rack shaft coaxial type brushless motor in thisembodiment;

[0029]FIG. 3 is a view showing the configuration in FIG. 2, cut off bythe line III-III as viewed in the arrow direction;

[0030]FIG. 4 is an enlarged view of a IV portion of the configuration inFIG. 2;

[0031]FIG. 5 is a sectional view of a side end of a ball screw nut,showing a modified example of this embodiment;

[0032]FIG. 6 is a sectional view taken in an axial direction, showingthe periphery of the rack shaft coaxial type brushless motor in theelectrically driven power steering apparatus in a second embodiment;

[0033]FIG. 7 is a sectional view taken in the axial direction, showingthe periphery of the rack shaft coaxial type brushless motor in a thirdembodiment;

[0034]FIG. 8 is a perspective view showing the ball screw nut and arotor of a motor in separation;

[0035]FIG. 9 is a perspective view showing a modified example of thisembodiment;

[0036]FIG. 10 is a sectional view taken in an axial direction, showingthe periphery of the rack shaft coaxial type brushless motor in theelectrically driven power steering apparatus in a fourth embodiment; and

[0037]FIG. 11 is a sectional view taken in an axial direction, showingthe periphery of the rack shaft coaxial type brushless motor in theelectrically driven power steering apparatus in a fifth embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

[0038] A first embodiment of the invention of the present applicationwill hereinafter be described in depth with reference to the drawings.FIG. 1 is a schematic diagram showing an electrically driven powersteering apparatus in the embodiment of the present invention. Referringto FIG. 1, a steering wheel 1 is connected to an upper side end of asteering shaft 2.

[0039] A lower side end of the steering shaft 2 is connected via auniversal joint 4 to an upper side end of a lower shaft 5, and further alower side end of the lower shaft 5 is connected via a universal joint 6to an upper side end of a pinion shaft 7. An unillustrated pinion isconnected to a lower side end of the pinion shaft 7 and meshes with rackteeth of a ball screw shaft, i.e., a rack shaft 22 (FIG. 22). A rackshaft coaxial type 5-phase rectangular wave driving type brushless motor23 is disposed in a mode that will be explained later on in a rackhousing 8 through which the rack shaft 22 is inserted.

[0040] A torque sensor 3 is disposed in the vicinity of the pinion shaft7 and detects a steering torque transferred to the pinion shaft 7. Thetorque sensor 3 is structured to converts it into, for example, adisplacement of torsion angle of a torsion bar (not shown) interposedbetween two-divided pinion shafts 7 and to magnetically mechanicallydetect this displacement of torsion angle. Accordingly, the torquesensor 3, when an operator steers the steering wheel 1, outputs, to acontroller 13, torque detection signals Tv consisting of analog voltagescorresponding to a magnitude of steering force and a steering direction.

[0041] Namely, the torque sensor 3, for instance, when the steering isin a neutral state, outputs a predetermined neutral voltage as thetorque detection signal Tv. The torque sensor 3, if the steering wheel 1is turned rightwards from this neutral state, outputs a voltage largercorresponding to a steering torque at this time than the neutralvoltage, and, if the steering wheel 1 is turned leftwards, outputs avoltage smaller corresponding to a steering torque at this time than theneutral voltage.

[0042] The controller 13 is provided for controlling a drive of themotor 23 and controlling a steering assistive force for a steeringsystem. The controller 13 is supplied with the power from a battery 16mounted in a car and is thereby operated. A negative pole of the battery16 is grounded, and its positive pole is connected to the controller 13via an ignition switch 14 for starting an engine and a fuse 15 a anddirectly connected to the controller 13 via a fuse 15 b. The powersupplied via this fuse 15 b is used for backing up a memory. Thecontroller 13 is capable of drive-controlling the brushless motor 23 onthe basis of the torque detection signals Tv transmitted from the torquesensor 3 and, for example, car speed detection signal Vp transmittedfrom a car speed sensor 17 disposed on an output shaft of anunillustrated transmission.

[0043]FIG. 2 is a sectional view taken in an axial direction, showingthe periphery of the rack shaft coaxial type brushless motor in thefirst embodiment. The housing, i.e., the rack housing 8 is fixed to anunillustrated car body through a bracket 21. The rack shaft 22 isinserted through within the rack housing 8 and connected at its two sideends to tie rods 9 (FIG. 1), 10. The tie rods 9, 10 are connected to anunillustrated steering mechanism.

[0044] A stator 23 b taking a shape of circular tube is fixed within therack housing 8, and a coil 23 a constructed of a plurality of segmentswound on a part of the stator 23 b, is also provided therein. A rotor 23c taking a shape of long and thin circular tube is so provided as toinsert into the stator 23 b. A cylindrical magnet 23 d for driving isprovided facing to the stator 23 a on an outer periphery of the rotor 23c. The drive magnet 23 d is magnetized to for forming N- and S-polesalternately in the circumferential direction. The rack shaft 22 extendswithin the rotor 23 c. Note that the stator 23 b, the coil 23 a, therotor 23 c and the drive magnet 23 d constitute the brushless typeelectric motor 23.

[0045] The rotor 23 c is so supported as to be rotatable along withinthe rack housing 8 by a ball bearing and an unillustrated bearing. Notethat a rotor for detecting a phase is, though not illustrated, attachedto the outer periphery of the rotor 23 c. This phase detection rotordetects a polarity of the drive magnet 23 d and is therefore disposed tohave a given correlation with the polarity. This polarity phase isdetected by a resolver R (FIG. 6) disposed adjacently to a positiondetecting magnet, and an electric signal indicating this polarity phaseis outputted to the controller 13.

[0046] The controller 13 supplies and distributes the electric currentsequentially to the segments of the respective coils 23 a divided in therotational direction, and as a result the brushless motor 23 isdrive-controlled to generate a predetermined rotational output.

[0047] A left side end of the rotor 23 c is connected to a right sideend of a ball screw nut 29 taking substantially a circular tube-likeshape. The ball screw nut 29 has an internal helical screw groove 29 bformed inside, and the internal screw groove 29 b forms a rolling pathfacing to an external screw groove 22 a formed in a left side portion ofthe rack shaft 22, wherein a plurality of balls 30 are accommodated inthe rolling path.

[0048] The balls are used for reducing a frictional force generated whenthe ball screw nut 29 and the rack shaft 22 relatively rotate. Note thatthe ball screw nut 29 has a circulation path 29 c, wherein the balls 30can be circulated through the circulation path 29 c when the ball screwnut 29 rotates.

[0049] A left side end of the ball screw nut 29 is so supported by anangular contact type ball bearing 25 as to be rotatable along within thehousing 8.

[0050]FIG. 3 is a view showing the configuration in FIG. 2, cut off bythe line III-III as viewed in the arrow direction. FIG. 4 is an enlargedview of an IV portion of the configuration in FIG. 2. As illustrated inFIG. 4, a male thread 29 a is formed on the outer periphery of the leftside end of the ball screw nut 29. A right side end of a cylindricalpresser member 31 formed with a female thread 31 a meshing with the malethread 29 a, is brought into contact with an inner ring of the bearing25.

[0051] A distal end (a left side end in FIG. 4) of the presser member 31is formed with a thin cylindrical portion 31 b extending outward in theaxial direction. Note that the outer periphery of the presser member 31has, as shown in FIG. 3, four notches 31 c formed at an equal intervalin the peripheral direction. The notches 31 c are used for rotating thepresser member 31 in a way that engages with a till (not shown).

[0052] Next, an operation in this embodiment will be explained withreference to the drawings. Referring to FIG. 1, supposing that thevehicle travels straight and the steering force is not yet inputted tothe rack shaft 22 from the steering wheel 1, the torque detection signalTv outputted from the torque sensor 3 is zero or a low value, and hencethe controller 13 does not perform the rotational control of thebrushless motor 23. Accordingly, the present electrically driven powersteering apparatus is in a state of not outputting the assistivesteering force.

[0053] On the other hand, when the vehicle turns a curve, the steeringwheel 1 is steered, and the steering force is transferred to the rackshaft 22. Therefore, the torque sensor 3 outputs the torque detectingsignal Tv corresponding to a steering torque, and the controller 13rotates the rotor 23 c of the brushless motor 23 with a proper torque ina way that takes a detection signal Vp transmitted from the speed sensor17 into consideration. When the rotor 23 c rotates, the ball screw nut29 also rotates with the result that the rack shaft 22 moves in the leftor right direction, thereby generating the assistive steering force.

[0054] By the way, the presser member 31 is screwed to the ball screwnut 29 in order to prevent the ball bearing 25 from coming off the ballscrew nut 29. If the presser member 31 is fastened with a strong torquefor increasing an initial axial force in order to prevent a slack of thepresser member 31, however, it follows that the circulation path 29 cformed within the ball screw nut 29 deforms. This might cause a declineof function of the ball screw nut 29.

[0055] A contrivance to obviate this problem is, according to thisembodiment, that the presser member 31 is screwed to the ball screw nutwith a toque small enough not to cause the deformation of the circularpath 29 c, and thereafter the thin cylindrical portion 31 b of thepresser member 31 is caulked (C) in the radial direction and made todeform so as to be pressed strong against the male thread 29 a of theball screw nut 29. The presser member 31 is thereby so connected as tobe impossible to a relative rotation with respect to the ball screw nut29, and does not slacken even when the strong force is transferred fromthe rack shaft 22. In this embodiment, the caulking (C) configures ahindering means.

[0056] On the other hand, according to this embodiment, a backlash ofthe ball 30 within the rolling path in the ball screw nut 29 is easilyadjusted. For instance, after incorporating the ball bearing 25 into theball screw nut 29, the presser member 31 is screwed to the side end ofthe ball screw nut 29, and the presser member 31 is rotated by anunillustrated tool till a proper pre-load is given.

[0057] When the proper pre-load is given, the thin cylindrical portion31 b of the presser member 31 is caulked (C) in the radial direction.The presser member 31 is thereby so connected as to be impossible of therelative rotation with respect to the ball screw nut 29, whereby theball 30 with the backlash can be stably retained within the ball screwnut 29.

[0058]FIG. 5 is a sectional view of the side end of the ball screw nut,showing a modified example of this embodiment. Referring to FIG. 5, apresser member 131 and a ball screw nut 129 are formed with holes 131 a,129 a penetrating therethrough in the radial directions. The pressermember 131 is not formed with the thin cylindrical portion. Other pointsare the same as those in the embodiment discussed above, and hence theirrepetitive explanations are omitted.

[0059] In this modified example, after a proper pre-load is given byrotating the presser member 131, the holes 131 a, 129 a are filled witha molten resin 132. When the resin 132 defined as a hindering means issolidified, it follows that the presser member 131 is fixed to the ballscrew nut 129 and does not slacken by dint of its shearing force andfrictional force even when the strong force is transferred from the rackshaft 22, whereby the ball 30 with the backlash can be stably retainedwithin the ball screw nut 129.

[0060] Note that if desiring a decomposition of the ball bearing 25, theresin 132 is sheared when the presser member 131 is rotated by a strongforce, and therefore the presser member 131 and the ball bearing 25 canbe removed from the ball screw nut 129. When reassembling thecomponents, the resin 132 is removed from the holes 129 a, 139 a, andthe components may be assembled in the same steps.

[0061]FIG. 6 is a sectional view taken in an axial direction, showingthe periphery of the rack shaft coaxial type brushless motor in theelectrically driven power steering apparatus in a second embodiment.Only a difference in the second embodiment is a configuration of theperiphery of the ball screw nut. Therefore, the discussion will befocused on this configuration, and the same components as those in thefirst embodiment are marked with the same numerals of which theexplanations are omitted. A rack housing 208 constructed of asmall-diameter portion 208 a and a large-diameter portion 208 b is fixedto the unillustrated car body with an unillustrated bracket. The rackshaft 22 is inserted into the large-diameter portion 208 a of the rackhousing 208 and connected at its two side ends to tie rods 9 (FIG. 1),10. The tie rods 9, 10 are connected to an unillustrated steeringmechanism.

[0062] A rotor 123 c of a brushless motor 123 inserted into the rackhousing 208 is so supported by a bearing 226 as to be rotatable alongwithin the rack housing 208. A left side end of the rotor 123 c isspline-engaged with a right side end of a ball screw nut 229 takingsubstantially a shape of circular tube, and the rotor and the screw nutare, though integrally rotatable, relatively movable in the axialdirection. A resin is coated over at least one of a toothed surface of afemale spline 229 d formed in the ball screw nut 229 (or the rotor 123c) and a toothed surface of a male spline 123 d formed on the rotor 123c (or the ball screw nut 229) which are spline-engaged with each other.With this contrivance, when an impact is exerted such as impinging upona steering stopper, the resin coating absorbs the impact, thus schemingto prevent an emission of butting noises. The ball screw but 229 has aninternal helical screw groove 229 b formed inside, and the internalscrew groove 229 b forms a rolling path facing to an external screwgroove 22 a formed in a left side portion of the rack shaft 22, whereina plurality of balls 30 are accommodated in the rolling path.

[0063] The balls 30 are used for reducing a frictional force generatedwhen the ball screw nut 229 and the rack shaft 22 relatively rotate.Note that the ball screw nut 229 has a circulation path 229 c, whereinthe balls 30 can be circulated through the circulation path 229 c whenthe ball screw nut 229 rotates.

[0064] An angular contact type bearing 251 for rotatably supporting theball screw nut 229 is provided along an inner periphery of thesmall-diameter portion 208 a of the rack housing 208 via a thincylindrical slide bush 231. The bearing 251 is constructed of an outerring 251 a, a couple of inner rings 251 b, 251 c and two trains of balls251 d disposed between the two rings.

[0065] A couple of flanged cylindrical core metals 252 a, 252 b eachassuming an L-shape in section on one side are disposed in a way ofcoming into contact with the both side ends of the outer ring 251 a ofthe bearing 251. A ring-shaped elastic member 235 a is disposed betweenthe left-sided core metal 252 a and a spacer fitted to thesmall-diameter portion 208 a. On the other hand, a ring-shaped elasticmember 235 b is disposed between the right-sided core metal 252 b and anut 233 screwed to the small-diameter portion 208 a.

[0066] The inner rings 251 b, 251 c of the bearing 251 are fitted to theball screw nut 229 through a nut 227 screwed to the outer periphery ofthe left side end of the ball screw nut 229.

[0067] According to this embodiment, the nut 227 is screwed to the ballscrew nut 229 with a torque small enough not to cause a deformation ofthe circulation path 229 c, and thereafter the thin cylindrical portion227 b extending in the axial direction from the nut 227 is caulked inthe radial direction so as to deform in a way of being pressed strongagainst the outer periphery of the ball screw nut 229. The nut 227 isthereby so connected as to be impossible of a relative rotation withrespect to the ball screw nut 229, and does not slacken even when thestrong force is transferred from the rack shaft 22.

[0068] According to this embodiment, if a large impact is exerted on therack shaft 22 serving as the ball screw shaft such as an impingementupon the steering stopper and so on, the ball screw nut 129 is permittedto move together with the bearing 251 in the axial direction while beingsupported by the slide bush 231. In such a case, however, in addition tothe above-described effect of the resin coating over the spline teeth,the elastic members 235 a, 235 b can effectively absorb the impact andrestrain the emission of the butting noises.

[0069] The present invention has been discussed so far by way of theembodiments. The present invention should not, however, be construed asbeing limited to the embodiments described above and can be, as a matterof course, properly modified and improved. For example, the hinderingmeans for preventing the presser member from slackening may be a filler,coated between the male thread and the female thread, for increasing theinter-thread frictional force.

[0070]FIG. 7 is a sectional view taken in an axial direction, showingthe periphery of the rack shaft coaxial type brushless motor in theelectrically driven power steering apparatus in a third embodiment. Arack housing 308 constructed of a small-diameter portion 308 a and alarge-diameter portion 308 b is fixed to the unillustrated car body witha bracket 321 formed integrally with the small-diameter portion 308 a. Arack shaft 322 is inserted into the large-diameter portion 308 a of therack housing 308 and connected at its two side ends to tie rods 9 (FIG.1), 10. The tie rods 9, 10 are connected to an unillustrated steeringmechanism. Note that the rack shaft 22 constitutes a ball screw shaft.

[0071] A stator 323 b taking a shape of circular tube is fixed withinthe rack housing 308, and a coil 323 a constructed of a plurality ofsegments wound on a part of the stator 323 b, is also provided therein.A rotor 323 c taking a shape of long and thin circular tube is soprovided as to insert into the stator 323 b. A cylindrical magnet 323 dfor driving is provided facing to the stator 323 a on an outer peripheryof the rotor 323 c. The drive magnet 323 d is magnetized to for formingN- and S-poles alternately in the circumferential direction. The rackshaft 322 extends within the rotor 323 c. Note that the stator 323 b,the coil 323 a, the rotor 323 c and the drive magnet 323 d constitutethe brushless type electric motor 323.

[0072] The rotor 323 c is so supported as to be rotatable along withinthe rack housing 308 by bearings 326 a, 326 b. A resolver 327 fordetecting a polarity phase is attached to the outer periphery of therotor 23 c in the vicinity of the bearing 326 b. This resolver 327 isdisposed to exhibit a given correlation with the polarity in order todetect the polarity of the drive magnet 323 d. An electric signalindicating the polarity phase detected by the resolver 327 is outputtedto the controller 13 (FIG. 1) via an unillustrated wire.

[0073] The controller 13 supplies and distributes the electric currentsequentially to the segments of the respective coils 323 a divided inthe rotational direction, and as a result the brushless motor 323 isdrive-controlled to generate a predetermined rotational output.

[0074] A left side end of the rotor 323 c engages with a right side endof a ball screw nut 329 taking substantially a circular tube-like shapein a mode that will hereinafter be explained. The rotor 323 c and theball screw nut 329 rotate integrally. The ball screw nut 329 has aninternal helical screw groove 329 b formed inside, and the internalscrew groove 329 b forms a rolling path facing to an external screwgroove 322 a formed in a left side portion of the rack shaft 322,wherein a plurality of balls 330 are accommodated in the rolling path.

[0075] The balls 330 are used for reducing a frictional force generatedwhen the ball screw nut 329 and the rack shaft 322 relatively rotate.Note that the ball screw nut 329 has a tube 329 c serving as acirculation path formed inside, wherein the balls 330 can be circulatedthrough the tube 329 c when the ball screw nut 329 rotates. Acylindrical member 329 d having a tube holding function and a functionof preventing a leak of grease.

[0076] A left side end of the ball screw nut 329 is so supported by a4-point contact type ball bearing 325 as to be rotatable with respect tothe small-diameter portion 308 a of the rack housing 308 as well asbeing regulated in its position in the axial direction. A right side endof the ball screw nut 329 is so supported by a ball bearing 328 as to berotatable with respect to the small-diameter portion 308 a of the rackhousing 308.

[0077] A cylindrical member 331 having an inward flange 331 a is screwedto a left side end of the small-diameter portion 308 a of the rackhousing 308. A bellow-shaped dust-proof boot 332 connects an outerperiphery of the cylindrical member 331 to an outer periphery of the tierod 10. A rack stroke damper 333, which is composed of a rubber or resinand has a groove formed in its outer periphery to get easy to deform, isfitted facing to the flange 331 a inwardly of the cylindrical member 331by use of a presser plate 334 taking substantially an L-shape insection. Even if the rack shaft 322 energetically displaces and aswollen side end 322 b of the rack shaft 322 impinges upon the presserplate 334, the impingement of the side end 322 b is damped by the rackstroke damper 333 disposed on the underside of the presser plate 334,thereby making it possible to prevent damages to the ball screw nut 329,the shaft 322 and the bearings 325, 328.

[0078]FIG. 8 is a perspective view showing the ball screw nut 329 andthe rotor 323 c of the motor 323 in separation. Referring to FIG. 8,four rectangular notches 329 e are formed at an equal interval in theperipheral direction in the side end portion of the ball screw nut 329.On the other hand, rectangular protrusions 323 e are formed at an equalinterval in the peripheral direction on the side end portion, facingthereto, of the rotor 323 c. A width (a length in the peripheraldirection) of the notch 329 e is set larger than a width (a length inthe peripheral direction) of the protrusion 323 e.

[0079] An elastic member 335 is disposed between the ball screw nut 329and the rotor 323 c. The elastic member 335 is constructed of acylindrical core metal 335 a, a rubber or resinous flange portion 335 bwelded to the outer periphery of the core metal 335 a, and two elasticportions (elastic members) 335 c each defined as substantially aC-shaped protrusion. An outside diameter of the core metal 335 a isslightly smaller than an inside diameter of each of the ball screw nut329 and of the rotor 323 c. The flange portion 335 b assumes adiscontinuous configuration in the peripheral direction, correspondingto the protrusions 323 e of the rotor 323 c, and the elastic portions335 c are connected in a state where the (two) discontinuous portions(among the four discontinuous portions) of the flange portion 335 b areshifted in the axial direction.

[0080] When the ball screw nut 329 and the rotor 323 c are connectedthrough the elastic members 335 serving as a connection member, the twoface-to-face protrusions 323 e of the rotor 323 c engage with thenotches 329 e of the ball screw nut 329 with almost no clearance in away that interposes the elastic portion 335 c therebetween. On the otherhand, the remaining two protrusions 323 e of the rotor 323 c engageswith the notches 329 e of the ball screw nut 329 with a predeterminedclearance in the peripheral direction without any interposition of theelastic portion 335 c. Note that the flange portion 335 b is interposedbetween the end surface of the ball screw nut 329 and the end surface ofthe rotor 323 c and hinders a direct contact of the ball screw nut 329and the rotor 323 c.

[0081] Next, an operation in this embodiment will be explained withreference to the drawings. Referring to FIG. 1, supposing that thevehicle travels straight and the steering force is not yet inputted tothe rack shaft 322 FIG. 7) from the steering wheel 1, the torquedetection signal Tv outputted from the torque sensor 3 is a neutralvoltage or substantially neutral voltage, and hence the controller 13does not perform the rotational control of the brushless motor 323 (FIG.7). Accordingly, the present electrically driven power steeringapparatus is in a state of not outputting the assistive steering force.

[0082] On the other hand, when the vehicle turns a curve, the steeringwheel 1 is steered, and the steering force is transferred to the rackshaft 322. Therefore, the torque sensor 3 outputs the torque detectingsignal Tv corresponding to a steering torque, and the controller 13rotates the rotor 323 c of the brushless motor 323 with a proper torquein a way that takes a detection signal Vp transmitted from the speedsensor 17 into consideration. When the rotor 323 c rotates, the ballscrew nut 329 also rotates with the result that the rack shaft 322 movesin the left or right direction, thereby generating the assistivesteering force.

[0083] Herein, if the traveling wheel collides with a paved step of thesidewalk in the vicinity of the neutral position where the rack strokedamper 333 does not function, the impact is transferred to the rackshaft 322. In such a case, the elastic member 335 torsionally deformsand can thus absorb the impact. Further, when the ball screw nut 329 andthe rotor 323 c relatively rotate through predetermined or larger angleswith the torsional deformation of the elastic member 335, the notch 329e as a recessed portion of the elastic portion 335 c and the protrusion323 e as a protrusion, which function as a displacement limiter, arebrought into contact with each other, thus preventing a damage to theelastic member 335 by limiting a further torsional deformation of theelastic member 335.

[0084] Note that a natural oscillation frequency of torsionaloscillations in the system consisting of the rotor 323 c and the ballscrew nut 329 including the elastic member 335, is 5 Hz or higher,preferably 7 Hz or higher, more preferably 8 Hz or higher so that theresonance does occur in the system as a control system.

[0085]FIG. 9 is a perspective view showing a modified example of thisembodiment. Referring to FIG. 9, a side end of a ball screw nut 329′ hasrectangular protrusions 329 e′ formed at an equal interval in theperipheral direction. On the other hand, a side end, facing thereto, ofa rotor 323 c′ has rectangular protrusions 323 e′ formed at an equalinterval in the peripheral direction.

[0086] An elastic member 335′ is disposed between the ball screw nut329′ and the rotor 323 c′. The elastic member 335′ is constructed of acylindrical core metal 335 a′, a rubber or resinous flange portion 335b′ extending over the entire outer periphery of the core metal 225 a′ inthe peripheral direction at the center thereof, and a rubber or resinousengagement portion (elastic member) 225 c′ with its fingers alternatelyextending on both sides in the axial direction, this engagement portionbeing integral with the flange portion 335 b′. An outside diameter ofthe core metal 335 a′ is slightly smaller than an inside diameter ofeach of the ball screw nut 329′ and of the rotor 323 c′.

[0087] When the ball screw nut 329′ and the rotor 323 c′ are connectedthrough the elastic members 335′, the protrusion 329 e′ of the ballscrew nut 329′ engages with the engagement portion 335 c′ in a way thatenters with almost no space, while the protrusion 323 e′ of the rotor323 c′ also engages with the engagement portion 335 c′ in a way thatenters with almost no space. At this time, the flange portion 335 b′ isinterposed between the end surface of the ball screw nut 329′ and theend surface of the rotor 323 c′ and hinders a direct contact of the ballscrew nut 329′ and the rotor 323 c′.

[0088] In this modified example, if the traveling wheel collides withthe paved step of the sidewalk and the impact is transferred to the rackshaft 322, the engagement portion 335 c′ of the elastic member 335′elastically deforms, thereby absorbing this impact.

[0089]FIG. 10 is a sectional view taken in an axial direction, showingthe periphery of the rack shaft coaxial type brushless motor in theelectrically driven power steering apparatus in a fourth embodiment.Only a difference in the fourth embodiment is a configuration of theperiphery of the ball screw nut. Therefore, the discussion will befocused on this configuration, and the same components as those in thefirst embodiment are marked with the same numerals of which theexplanations are omitted. A rack housing 408 constructed of asmall-diameter portion 408 a and a large-diameter portion 408 b is fixedto the unillustrated car body with an unillustrated bracket. The rackshaft 322 is inserted into the large-diameter portion 408 a of the rackhousing 408 and connected at its two side ends to tie rods 9 (FIG. 1),10. The tie rods 9, 10 are connected to an unillustrated steeringmechanism.

[0090] A rotor 423 c of a brushless motor 423 is so supported bybearings 326 a, 326 b as to be rotatable along within the rack housing408. A left side end of the rotor 423 c is spline-engaged with a rightside end of a ball screw nut 429 taking substantially a shape ofcircular tube, and the rotor and the screw nut thereby integrallyrotate. A resin is coated over at least one of toothed surfaces offemale and male splines meshing with each other, thereby scheming toprevent an emission of butting noises. The ball screw but 429 has aninternal helical screw groove 429 b formed inside, and the internalscrew groove 429 b forms a rolling path facing to an external screwgroove 322 a formed in a left side portion of the rack shaft 322,wherein a plurality of balls 330 are accommodated in the rolling path.

[0091] The balls 330 are used for reducing a frictional force generatedwhen the ball screw nut 429 and the rack shaft 322 relatively rotate.Note that the ball screw nut 429 has a circulation path 429 c formedinside, wherein the balls 330 can be circulated through the circulationpath 429 c when the ball screw nut 429 rotates.

[0092] A rubber or resinous damping member 440 taking a configurationcorresponding to the outer periphery of the rack shaft 322 is so fittedto the inner periphery of the left side end of the ball screw nut 429 asto come into contact with the outer periphery of the rack shaft 322. Forexample, if the rack shaft 322 is oscillated through the traveling wheelon the rugged surface of the road, the noises are generated when theballs 330 bounce by a backlash between the rolling surface and the ball330 and impinge on the rolling surface. It is, however possible torestrain the emission of the noises by providing the damping member 440to restrain the oscillations of the rack shaft 322 due to theimpingement of the balls 330. Further, an inside-diametrical portion ofthe damping member 440 is formed with a male thread groove engaging withan external thread groove of the rack shaft 322, and this has aso-called sealing function of preventing the grease in the ball screwnut 429 from leaking outside.

[0093] The outer periphery in the vicinity of the right side end of theball screw nut 429 is so supported by a plurality of trains of angularcontact ball bearings 425 as to be rotatable with respect to thesmall-diameter portion 408 a of the rack housing 408 and to be regulatedin its position in the axial direction. On the other hand, the outerperiphery of the left side end of the ball screw nut 429 is so supportedby a roller bearing 426 as to be rotatable with respect to thesmall-diameter portion 408 a of the rack housing 408.

[0094] An inner ring 425 b of the angular contact ball bearing 425 isfitted by a nut 427 screwed to the outer periphery of the ball screw nut429. The nut 427 will be explained later on. An outer ring 425 a of theangular contact ball bearing 425 is attached to the inner periphery ofthe small-diameter portion 408 a through a thin cylindrical slide bush431. Each of elastic members 435 disposed in a state of being broughtinto contact with both sides of the outer ring 425 a includes a flangedcylindrical core metal 435 a taking substantially an L-shape in sectionon one side and fixed to the inner periphery of the small-diameterportion 408 a, and a rubber or resinous elastic member 435 b. Note thata thread member 433 coming into contact with the right-sided elasticmember 435 b is so fitted to the small-diameter portion 408 a as to beadjustable in order to adjust an interval between the elastic members435.

[0095] A bellow-shaped dust-proof boot 332 connects an outer peripheryof the left side end of the small-diameter portion to an outer peripheryof the tie rod 10. A rubber or resinous rack stroke damper 333 having agroove formed in its outer periphery to get easy to deform, is fittedfacing to a flange 408 c of the small-diameter portion 408 a by use of aflanged cylindrical presser plate 334 taking substantially an L-shape insection on one side. Even if the rack shaft 322 energetically displacesand a swollen side end 322 b of the rack shaft 322 impinges upon thepresser plate 334, the impingement of the side end 322 b is damped bythe rack stroke damper 333 disposed on the underside of the presserplate 334, thereby making it possible to prevent damages to the ballscrew nut 429 and the bearing 425.

[0096] If the traveling wheel collides with the paved step of thesidewalk in the vicinity of the neutral position where the rack strokedamper 333 does not function and the impact is transferred to the rackshaft 322, the elastic portion 435 b of the elastic member 435elastically deforms in the axial direction, whereby the impact can beabsorbed.

[0097] Note that when the ball screw nut 429 and the rotor 423 crelatively rotate through predetermined angles, further rotationsthereof be, it is preferable, hindered by unillustrated stoppers. Thisis because, with this contrivance, a damage to the elastic portion 435 bcan be prevented by restraining an excessive deformation of the elasticportion 435 b.

[0098] Moreover, according to this embodiment, the nut 427 is screwed tothe ball screw nut 429 with the torque small enough not to cause thedeformation of the circulation path 429 c, and thereafter the thincylindrical portion 427 a extending in the axial direction from the nut427 is caulked in the radial direction so as to deform in a way of beingpressed strong against the outer periphery of the ball screw nut 429.The nut 427 is thereby so connected as to be impossible of a relativerotation with respect to the ball screw nut 429, and does not slackeneven when the strong force is transferred from the rack shaft 322.

[0099]FIG. 11 is a sectional view taken in an axial direction, showingthe periphery of the rack shaft coaxial type brushless motor in theelectrically driven power steering apparatus in a fifth embodiment. Onlya difference in the fourth embodiment is a configuration of theperiphery of the ball screw nut. Therefore, the discussion will befocused on this configuration, and the same components as those in thefirst embodiment are marked with the same numerals of which theexplanations are omitted. A rack housing 508 constructed of asmall-diameter portion 508 a and a large-diameter portion 508 b is fixedto the unillustrated car body with an unillustrated bracket. The rackshaft 322 is inserted into the large-diameter portion 508 a of the rackhousing 508 and connected at its two side ends to tie rods 9 (FIG. 1),10. The tie rods 9, 10 are connected to an unillustrated steeringmechanism.

[0100] A rotor 423 c of a brushless motor 423 is so supported bybearings 526 as to be rotatable along within the rack housing 508. Aleft side end of the rotor 423 c is spline-engaged with a right side endof a ball screw nut 529 taking substantially a shape of circular tube,and the rotor and the screw nut thereby integrally rotate. A resin iscoated over at least one of toothed surfaces of female and male splinesmeshing with each other, thereby scheming to prevent an emission ofbutting noises. The ball screw but 529 has an internal helical screwgroove 529 b formed inside, and the internal screw groove 529 b forms arolling path facing to an external screw groove 322 a formed in a leftside portion of the rack shaft 322, wherein a plurality of balls 330 areaccommodated in the rolling path.

[0101] The balls 330 are used for reducing a frictional force generatedwhen the ball screw nut 529 and the rack shaft 322 relatively rotate.Note that the ball screw nut 529 has a circulation path (not shown)formed inside, wherein the balls 330 can be circulated through thecirculation path 429 c when the ball screw nut 529 rotates.

[0102] A bearing 551 for rotatably supporting the ball screw nut 529 isdisposed on an inner periphery f the small-diameter portion 508 a of therack housing 508 through a thin cylindrical slide bush 531. The bearing551 is constructed of an outer ring 552 a, a couple of inner rings 551b, 551 c and two trains of balls 551 d disposed between the two rings.

[0103] A flanged cylindrical spacer 552 a taking an L-shape in sectionon one side and an elastic member 535 a are disposed between a left sideend of the outer ring 551 a of the bearing 551 and the small-diameterportion 508 a. A flanged cylindrical spacer 552 b taking an L-shape insection on one side and an elastic member 535 b are disposed between aright side end of the outer ring 551 a of the bearing 551 and a threadmember 533 screwed to the small-diameter portion 508 a. The inner rings551 b, 551 c of the bearing 551 are fitted to the ball screw nut 529 bya nut 527 screwed to the outer periphery of the left side end of theball screw nut 529. Note that an assembly width of the inner rings 551b, 551 c is set larger than an axial range of top holes for circulatingthe balls, thereby scheming to prevent the leak of the grease and thetops from being removed.

[0104] If the traveling wheel collides with the paved step of thesidewalk in the vicinity of the neutral position where the rack strokedamper 333 does not function and the impact is transferred to the rackshaft 322, any one of the elastic members 535 a, 535 b elasticallydeforms in the axial direction, whereby the impact can be absorbed. Notethat when each of the elastic members 535 a, 535 b deforms in excess ofa predetermined quantity, a side end of the spacer 552 a or 552 bserving as a displacement limiter is bottomed, thereby limiting furtherdeformations of the elastic members 535 a, 535 b.

[0105] Moreover, according to this embodiment, the nut 527 is screwed tothe ball screw nut 529 with the torque small enough not to cause thedeformation of the circulation path, and thereafter the thin cylindricalportion 527 a extending in the axial direction from the nut 527 iscaulked in the radial direction so as to deform in a way of beingpressed strong against the outer periphery of the ball screw nut 529.The nut 527 is thereby so connected as to be impossible of a relativerotation with respect to the ball screw nut 529, and does not slackeneven when the strong force is transferred from the rack shaft 322.

[0106] The present invention has been discussed so far by way of theembodiments. The present invention should not, however, be construed asbeing limited to the embodiments described above and can be, as a matterof course, properly modified and improved. For example, this embodimenthas exemplified the coaxial type brushless motor, however, the presentinvention can be of course applied to the electrically driven powersteering in which the motor shaft and rack shaft are independent, andthe drive torque transmission therebetween is performed by a reductiongear and so on. Further, the present invention is not confined to thisparallel geometry.

What is claimed is:
 1. An electrically driven power steering apparatuscomprising: a housing; a ball screw shaft extending within said housingand connected to a steering mechanism; an input shaft to which asteering force is inputted; an output shaft for receiving the steeringforce from said input shaft and outputting the steering force to saidball screw shaft; a torque sensor for detecting a torque transferredbetween said input shaft and said output shaft; a motor including arotor; and a ball screw nut for giving a force in an axial direction tosaid ball screw shaft by receiving a rotational force from said motor,wherein an elastic member deforming and thus capable of absorbing animpact inputted from the side of said ball screw shaft, is disposed on apower transmission route between said ball screw shaft and said rotor ofsaid motor.
 2. An electrically driven power steering apparatus accordingto claim 1, wherein said elastic member is disposed between said ballscrew nut and said rotor of said motor, and the impact inputted from theside of said ball screw shaft is absorbed by a torsional damper effect.3. An electrically driven power steering apparatus according to claim 6,wherein a displacement limiter for limiting a predetermined or largerquantity of deformation of said elastic member is provided andconstructed of a recessed portion formed in one of said rotor of saidmotor and said ball screw nut and a protruded portion formed on theother, and said protruded portion, when said elastic member deforms bythe predetermined quantity, engages with said recessed portion.
 4. Anelectrically driven power steering apparatus comprising: a housing; aball screw shaft extending within said housing and connected to asteering mechanism; an input shaft to which a steering force isinputted; an output shaft for receiving the steering force from saidinput shaft and outputting the steering force to said ball screw shaft;a torque sensor for detecting a torque transferred between said inputshaft and said output shaft; a motor including a rotor; and a ball screwnut for giving a force acting in an axial direction to said ball screwshaft by receiving a rotational force from said motor, wherein anelastic member deforming and thus capable of absorbing an impactinputted from the side of said ball screw shaft, is disposed on asupport portion of said ball screw nut.
 5. An electrically driven powersteering apparatus according to claim 4, wherein said elastic member isdisposed between said housing and a bearing for supporting said ballscrew nut so as to be rotatable with respect to said housing or betweensaid bearing and said ball screw nut.
 6. An electrically driven powersteering apparatus according to claim 5, wherein said bearing and saidhousing relatively move in the axial direction corresponding to theaxis-directional deformation of said elastic member, and there isprovided a displacement limiter for limiting a predetermined or largerquantity of deformation of said elastic member by limiting the relativemovements of said bearing and said housing in the axial direction.
 7. Anelectrically driven power steering apparatus according to any one ofclaims 1 through 6, wherein said rotor of said motor and said ball screwnut are connected by an engagement between a female spline and a malespline of which at least one toothed surface is coated with a resin. 8.An electrically driven power steering apparatus according to any one ofclaims 1 through 7, wherein said displacement limiter limits thepredetermined or larger quantity of deformation of said elastic memberat 40% or smaller of a maximum steering force exhibited by said motor.9. An electrically driven power steering apparatus according to any oneof claims 1 through 8, wherein a natural oscillation frequency of asystem constructed of said rotor, said ball screw nut and said elasticmember is set to 7 Hz or higher.
 10. An electrically driven powersteering apparatus comprising: a housing; a ball screw shaft extendingwithin said housing and connected to a steering mechanism; a motorhaving a rotor; a ball screw nut, connected to said rotor of said motor,for converting a rotational force of said rotor into a force acting inan axial direction and transferring the same force to said ball screwshaft; a bearing for supporting said ball screw nut to as to berotatable with respect to said housing; and a presser member, screwed tosaid ball screw nut, for thus pressing said bearing against said ballscrew nut, wherein said presser member includes a connection member forconnecting said presser member to said ball screw nut so that saidpresser member and said ball screw nut are unable to relatively rotate.11. An electrically driven power steering apparatus according to claim10, wherein said connection member connects said presser member to saidball screw nut so as to be unable to relatively rotate by use of ashearing force of a resinous material.
 12. An electrically driven powersteering apparatus according to claim 10, wherein said connection memberconnects said presser member to said ball screw nut so as to be unableto relatively rotate by use of a frictional force.
 13. An electricallydriven power steering apparatus according to any one of claims 10through 12, wherein said presser member is screwed to said ball screwnut and thus adjusts a backlash of the balls within said ball screw nut.